LAUSR

This paper reports the first use of a 3-D radar imaging for observing E-region field-aligned plasma irregularities (FAIs) in the midlatitude ionosphere. Five carrier frequencies equally spaced between 46.25 and 46.75 MHz were transmitted alternately with consecutive radar pulses, and 20 receivers were employed for receiving the radar echoes. The experiments were carried out using the middle and upper atmosphere radar (34.85°N, 136.10°E), with the radar beams steered to the geographic north and 6.6° north by east, at the zenith angles of 51.0° and 51.5°, respectively. The 2-D imaging with the echoes collected by the 19 of the 20 receivers revealed that in zonal dimension, the off-beam direction of arrival (DOA) of the echo region changed alternately between positive and negative values, which can be attributed to the highly localized FAIs drifting zonally through the radar beam. On the other hand, the off-beam DOA in meridional dimension was negative and positive, respectively, at higher and lower range locations, which were supposed to be due to the meridional drift component of FAIs. A combination of 5 frequencies and 19 receivers achieved a 3-D imaging of the FAI structures, illustrating small-scale FAI striations and bubblelike plasma structures in the radar volume. Moreover, declination of geomagnetic field line was examined from the imaged 3-D brightness distribution in the radar volume; the estimated declination of geomagnetic field line was in agreement with that computed from the International Geomagnetic Reference Field model in 2011. This paper has demonstrated some capabilities of radar imaging for ionospheric investigation.